1. Field of the Invention
This invention relates to a display element formed by a pair of parallel glass plates each having opposed electrodes formed thereon, and more particularly to a spacer for separating the plates by a predetermined distance and for making electrical connection between the parallel plates. The invention further relates to a method of producing a display element.
2. Description of the Prior Art
As is known, both luminescent and non-luminescent display elements are used today for representing information. The basic structure of various such types of display elements which work in accordance with different principles, consists of two plates arranged parallel to one another at a certain distance and covered partially with electrically conductive electrodes which are matched functionally to the information to be displayed. The type of display elements mentioned includes liquid crystal displays of different types, i.e. electrochromatic, electrophoretic and ferroelectric display elements, these types of display elements being represented in greater detail in the book "Nonemissive Electrooptic Displays" by A. R. Kmetz and F. K. von Willisen, Plenum Press 1976, page 201 et seq. Another common characteristic of the different types of display elements mentioned, which may be noted, is that the electrodes of both plates are connected electrically to a voltage or current source by which the information to be represented is made visible by driving opposed electrodes of the display element in the appropriate manner.
All these display elements also have the common feature that the plates forming their boundaries, mostly glass plates, have to be held at a certain distance from one another which corresponds in an optimum manner to the function of the display. This distance can be a few micrometers up to several hundred micrometers.
The thickness of the intermediate space formed by the two plates is of decisive significance for the operation, but especially also for the optical and electric quality and the service life of a display unit. For this reason it is desirable that this thickness deviates as little as possible from the nominal thickness over the whole area of a display unit as well as from display unit to display unit within a production run. As is known, the intermediate space formed by the two plates is closed off hermetically to the outside by cementing the peripheral plate edges together with an electrically insulating cement, approximately according to Swiss Patent 584,949. Due to the previously stated reasons, this cement ridge must have a thickness which is as uniform as possible and equal to the nominal thickness of the intermediate space, and there are no indications to this effect in the state of the art mentioned.
A certain improvement in accuracy can be achieved according to German Offenlegungsschrift No. 2,406,623 wherein mechanically stable particles, the diameter of which corresponds to the optimum distance between plates, is admixed to the cement or sealing compound. According to the process described in German Offenlegungsschrift No. 2,406,623, the plates are pressed together by suitable devices during the cementing process, the cement being in a liquid state, until they come to rest on the admixed particles and thus a minimum distance is guaranteed. This process has the disadvantage, however, that it is not possible to produce powders of an infinitely narrow particle size distribution resulting in a more or less wide spread in the plate spacing.
For this reason, as is known, for example, from U.S. Pat. No. 4,217,035, methods have been developed in which the powder is replaced by short pieces of glass fibers. Glass fibers have an exactly defined diameter and are better suited for producing display elements with tight thickness tolerances. In addition they are mechanically more stable due to their relatively long support line. The problem of obtaining an adequately accurate and constant distance between the plates could be solved with glass fibers, but unfortunately they simultaneously stand in the way of the solution to the following, very important problem.
As has been mentioned before, for operating a display device both the front and the back electrode must be provided with electric contacts.
This could be managed by bringing the corresponding electrode tracks out laterally on externally accessible plate areas which, however, is firstly very cumbersome and costly and, secondly, not always possible.
Thus it is extremely desirable, if not a necessity with respect to the application, that all the contact connections of the display element are located in one plane, that is the contact connections of, for example, the back electrode must also be conducted via an electrically conductive bridge to the plane of the contact tracks of the front electrode.
This electrically conductive bridge between the two electrodes can be produced, as for example in the said Swiss Pat. No. 584,949 or in German Offenlegungsschrift No. 2,555,822, by applying one or several contact points of conductive silver, conductive epoxy or a glass containing silver in the case of plates with glass-to-glass seals. In addition, attempts have been made to mix conductive powders such as small metal beads, graphite powder or also semi-conductive silicon carbide in with the cement itself. None of these methods, however, have brought the desired success since these electrically conductive powders have the same disadvantages as the solid particles already known for limiting the plate spacing from German Offenlegungsschrift No. 2,406,623. In addition, metallic powders in the appropriate fineness and selected diameters are very expensive.